1. Field of the Invention
The present invention relates generally to heat treatment of semiconductor wafers, and more specifically to vertical furnaces used in thermal curing of semiconductor wafers during fabrication.
2. Description of the Related Art
Semiconductor devices are formed from silicon wafers containing various circuitry defining the semiconductor device. During the formation of the circuitry on and in the silicon wafer, the wafers are processed through a plurality of fabrication operations to form, define, and refine the layers that make up the multi-layered structure. As is known, some layers are formed over wafers in the form of thin films that must be cured in order to carry out thermolytic reactions and/or remove solvents.
Vertical furnaces have been used in curing processes of semiconductor manufacture for many years. Typically, a vertical furnace is used in batch processing wherein 25 or more wafers at a time are inserted into a vertical furnace tube for heat treatment. Such heat treatment includes the curing of low k dielectric films applied to a surface of a semiconductor wafer. As an alternative to the batch processing of vertical or horizontal furnace tubes, semiconductor wafers are processed individually in curing modules.
FIG. 1A shows a typical prior art single wafer curing module 10. The curing module 10 includes a heating plate 12 and a cooling chamber 20. A semiconductor wafer 11 is positioned within a chamber 14 of the curing module 10 through a door 24 attached to an arm 24a. Once the wafer 11 is within the chamber 14, the door 24 seals the curing environment within. The semiconductor wafer 11 is positioned on lift pins 22 in the curing module 10 as shown. During a heating process, the wafer 11 is lowered onto heating plate standoffs 21 which are integral to the heating plate 12.
After the wafer 11 is heated, the wafer 11 must be cooled to prevent oxidation of the organic components of the film on the surface of the wafer 11 when the wafer is removed from the chamber 14 and once again exposed to the ambient atmosphere. The wafer 11 is cooled by first raising the wafer 11 on the lift pins 22 from the heating plate 12 towards a diffusion plate 18. The diffusion plate serves as a shower head to dispense the cooling medium of the cure module 10. The wafer 11 is typically cooled with water (H2O)-cooled nitrogen gas (N2) flowing through the cooling chamber 20 and dispersed by the diffusion plate 18 and over the wafer 11. Once the wafer 11 is cooled to an appropriate temperature at which the film will not oxidize, the wafer 11 is removed from the curing module 10 through door 24.
Common problems with the curing module 10 processing technique include thermal discontinuities on the surface of the wafer 11 resulting from the lift pins 22. Further, the curing module 10 is generally inefficient in performing both the heating and cooling operations of the curing process. By way of example, while the wafer 11 is being heated, heat rises from the heating plate 12 to the diffusion plate 18 which increases the surface temperature of the diffusion plate 18. The increased temperature of the diffusion plate 18 raises the temperature of the cooled nitrogen gas as it exits through the diffusion plate 18. The higher temperature of the nitrogen gas increases the cooling time of the wafer 11, thereby decreasing the overall efficiency of the curing process. Additionally, the cure module is a single-wafer process, consuming precious time and resources in the wafer fabrication process.
FIG. 1B shows a typical prior art vertical furnace unit 50. The vertical furnace unit commonly includes a heating element 52 which can surround or define the furnace cavity or furnace tube 54. Wafers 11 are staged in a cassette or boat 58 designed specifically for use in a vertical furnace unit 50. The boat 50 can be designed to hold a plurality of wafers 11 ranging from 25 to more than a hundred. The primary advantage of the vertical furnace unit for wafer curing is batch wafer processing.
In the illustrated vertical furnace unit 50, the wafer boat 58 is positioned on a pedestal 56 that lifts and withdraws the wafers 11 into and from the furnace tube 54. The pedestal 56 is commonly configured to provide a means for dispersing N2 while the wafers 11 are disposed within the furnace tube 54. Such dispersion can occur through the pedestal itself acting as a diffusion plate, or by way of providing plumbing or ducting to flow N2 around the wafers 11 and/or within the furnace tube 54.
One common alternative to the pedestal 56 is a wafer boat 58 that is on a gear-driven track configured to move the boat 58 into and out of the furnace tube 54. As an alternative to the vertical orientation of the vertical furnace unit 50, the unit can be configured to be oriented horizontally. In such an orientation, corresponding similar components as those described perform essentially the same functions as described to cure wafers.
There are numerous disadvantages to typical prior art vertical or horizontal furnace units. As true batch processors, vertical or horizontal furnace units that are configured to cure 25 or more wafers at a time are ill suited for single wafer processing such as used in cluster arrangements of spin coat tracks and the like. Further, the furnace tube can result in non-uniform curing of the surface of a wafer as the applied heat in the tube structure results in the periphery of the wafers heating faster and to a greater degree than the center of the wafers. Additionally, in situations in which the desired cure time is short (e.g., less than 15 minutes), the single point of entry into the furnace tube results in a first in/last out wafer curing process. The difference in time at the desired temperature causes a significant difference in the degree of cure experienced by the first wafer and the last wafer in a batch of wafers. In the case of low k dielectric films, the needed cure time can be as short as 3-5 minutes, and the prior art vertical or horizontal furnace units don""t provide the necessary high speed and continuous processing.
In view of the foregoing, there is a need for a cure furnace which is configured to efficiently and uniformly cure semiconductor wafers. The cure furnace should be efficient in low k dielectric applications in which high speed and continuous processing is required, and should be configured to integrate batch and single-wafer processing environments. The cure furnace should therefore be configured to avoid the problems of the prior art.
Broadly speaking, the present invention fills these needs by providing an in-line cure furnace configured to thermally cure films on semiconductor substrates while achieving high through-put, rapid processing, and precisely controlled thermal profiles. The present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several embodiments of the present invention are described below.
In one embodiment, an in-line cure furnace is disclosed. The in-line cure furnace includes a vertical furnace tube configured to thermally process a substrate, a hot plate configured to pre-heat the substrate for thermally processing in the vertical furnace tube, and a passageway connecting the hot plate to the vertical furnace tube. The in-line cure furnace further includes a cooling chamber and a passageway connecting the vertical furnace tube to the cooling chamber through which the substrate is transitioned from the vertical furnace tube to the cooling chamber.
In another embodiment, a method for thermally curing a substrate is disclosed. The method includes providing a substrate having a film requiring thermal curing and pre-heating the substrate on a hot plate. The substrate is then transitioned from the hot plate to a cassette disposed within a vertical furnace tube. The cassette is indexed to a next position to receive another substrate, and the method repeats until an entire cassette is filled with substrates. The substrates are then thermally cured in the vertical furnace tube before transitioning the substrates, one at a time, from the cassette to a cool plate, and removing the cooled substrate from the cool plate.
In still a further embodiment, a cure furnace system is disclosed. The cure furnace system includes a vertical furnace tube having a lower region, a middle region, and an upper region. A hot plate is connected to a lower passageway which is connected to the lower region of the vertical furnace tube. A cooling chamber is connected to the upper region of the vertical furnace tube, and a wafer transport is configured to transition semiconductor wafers from the lower region to the upper region of the vertical furnace tube.
In yet another embodiment, a cure furnace system is disclosed. The cure furnace system includes a pair of vertical furnace tubes with each vertical furnace tube having a lower region, a middle region, and an upper region. A hot plate is configured to a lower passageway which is connected to the lower region of each of the pair of vertical furnace tubes. A cooling chamber is configured to the upper regions of the pair of vertical furnace tubes, and a wafer cassette configured to hold substrates is disposed within each of the pair of vertical furnace tubes. The wafer cassette is configured to transition the substrates from the lower region of each vertical furnace tube to the upper region of each vertical furnace tube.
The advantages of the present invention are numerous. One notable benefit and advantage of the invention is the ability to achieve rapid and continuous thermal processing of substrates which is of particular advantage in the processing of low k dielectric films. The present invention provides for pre-heating of a substrate prior to entry into the furnace tube to rapidly bring the substrate to curing temperature. The substrates are transitioned from the hot plate to a wafer transport in the furnace tube, indexed into the furnace tube, cured, and removed to a cooling chamber in a continuous and efficient process that achieves high through-put and rapid curing.
Another benefit is precise temperature control of the wafer curing. The invention implements a first in/first out process for thermal curing within the furnace tube so each substrate is processed through uniform, consistent thermal curing. Each substrate is preheated on a hot plate to overcome prior art problems of non-uniform temperature rise over the entire substrate in a furnace tube. The furnace tube uses heated nitrogen to exclude oxygen and further maintain curing temperature, and implements temperature zones within the furnace tube to precisely shape the temperature profile for thermal cure.
An additional benefit is the ability to configure a pair of vertical furnace tubes separated by the hot plate in the lower regions and the cooling chamber in the upper regions. Using this configuration, the invention can achieve maximum through-put by inserting substrates to be cured into one furnace tube while removing substrates having been cured from the other furnace tube. The invention can thus achieve continuous, uniform, and high volume thermal curing of substrates.
Yet another benefit is the ease with which the present invention is incorporated into a cluster processing arrangement. Because the invention can achieve continuous processing and high through-put, proximity to additional substrate processing apparatus is essential for efficient implementation. The present invention utilizes a single robot to insert and extract substrates from the cure furnace system, and is easily incorporated into a modular cluster arrangement which can include spin-coat, pre-bake, and staging areas for high-volume processing.